The present invention relates to a servo accelerometer having a servo circuit for detecting a displacement amount of a pendulum and for generating a force balanced with a force applied to the pendulum by supplying a current corresponding to the detected displacement amount to a coil mounted on the pendulum.
An operation principle of a servo circuit of this type will be described with reference to a block diagram shown in FIG. 3.
In FIG. 3, reference numeral 1 denotes a mass; 2, a pendulum; 3, a displacement detector; 4, a servo amplifier; 5, a V/I converter; and 6, a torquer coil. A force F.sub.2 generated by the torquer coil 6 is adjusted so that F.sub.2 =0 when an acceleration .alpha. input to the servo accelerometer is zero. Since a force acting on the pendulum 2 is represented by a difference between a force F.sub.1 caused by the acceleration .alpha. acting on the mass m of the pendulum 2 and the force F.sub.2 generated by the torquer coil 6, the following equation is established. EQU F=F.sub.1 -F.sub.2 =m.alpha.-F.sub.2 ( 1)
The equation of motion of the pendulum 2 is: ##EQU1## where, D is a damping factor constant, and K is a spring constant of a flexure member.
By Laplace transformation of the above equation (2), a transfer function of the pendulum 2 can be obtained by the following equation. ##EQU2##
A displacement amount of the pendulum 2 is converted to a voltage V.sub.0 by the displacement detector 3 (conversion constant A.sub.0). The voltage V.sub.0 is multiplied with A.sub.1 to be a voltage V.sub.1 by the servo amplifier 4. The voltage V.sub.1 is converted to a current i by the V/I converter 5 (conversion constant A.sub.2). The current i is supplied to the torquer coil 6, and converted to a force F.sub.2 with a conversion constant A.sub.3. That is, by a negative feedback satisfying F.sub.1 =F.sub.2, i.e., generating a force F.sub.2 balanced with the force F.sub.1, the pendulum 2 is servo-controlled at a predetermined point.
In this case, since the current i is in proportion to the acceleration .alpha., an input acceleration can be detected on the basis of the value of current i.
It can be proved that the current i is in proportion to the acceleration .alpha. as follows. That is, referring to FIG. 3, a transfer function G between the acceleration .alpha. and the current i can be represented as follows. ##EQU3## If t=.infin., then S=0, and the following equation can be satisfied. ##EQU4## Equation (5) can be rewritten as follows when it is L assumed that K=0. ##EQU5## Since m and A.sub.3 are constants, the current i is in proportion to the acceleration .alpha..
A mechanical structure of the servo accelerometer on the basis of the above principle is shown in FIG. 4.
In FIG. 4, a pendulum 12 is swingably supported by a flexure member 11 in a direction of the input acceleration. Coils 14-1 and 14-2 are mounted on both the sides of the pendulum 12. Magnets 13-1 and 13-2 are arranged at both the sides of the pendulum 12 opposing to each other. That is, a torquer coil 17 is constituted by the magnets 13-1 and 13-2, and the coils 14-1 and 14-2. Since the coils 14-1 and 14-2 move in a magnetic field generated by the magnets 13-1 and 13-2 with the pendulum 12, by supplying a proper current i corresponding to a displacement amount x of the pendulum 12 to the coils 14-1 and 14-2, a force can be balanced with the input acceleration .alpha..
A slit plate 12-2 is secured on the lower end of the pendulum 12, and a slit 12-1 is formed in the slit plate 12-2. A displacement detection mechanism 18 is constituted by the slit 12-1, a two-split photodiode 15, and an LED 16. The position of the pendulum 12 is detected by the displacement detection mechanism 18. FIGS. 5 and 6 show a main part of the displacement detection mechanism 18. More specifically, at a zero point of the displacement detection mechanism 18, light of the LED 16 passing through the slit 12-1 is equally incident on photodiodes 15A and 15B. From this state, when the slit 12-1 moves in the direction of the arrow in FIG. 5, a difference appears between the incident light amounts of the photodiodes 15A and 15B. That is, the difference between the incident light amounts of the photodiodes 15A and 15B represents a displacement amount x and a displacement direction of the pendulum 12. When it is assumed that the difference of the incident light amounts can be obtained as an output voltage V.sub.0, ideal characteristics between displacement amounts and output voltages can be obtained as shown in FIG. 7. Considering the relationship of dimensions between the slit 12-1 and the photodiodes 15A and 15B, under the condition wherein Ls (a width of the slit 12-1)=Ld (a width of light-receiving planes of the photodiodes 15A and 15B), the largest detection range and the highest output voltage V.sub.0 can be obtained.
Note that, as a detection method of the displacement amount x of the pendulum 12, a method by a differential transformer, or a method using a change in capacitance can be used, as well as the method using the two-split photodiode described above. The methods by a differential transformer, or by change in capacitance are, however, methods of detecting a displacement amount by impedance measurement. As a result, an oscillator is required, so that it is disadvantageous to make a unit compact or reduce cost of the unit. That is, by the method using the two-split photodiode, a gain between the displacement amount x and the output voltage V.sub.0 is large, thus obtaining both economical and operational advantages.
FIG. 8 shows an electrical circuit arranged using the mechanical structure shown in FIG. 4. The LED 16 is connected to a power supply through a resistor R.sub.1 for setting a light amount of the LED 16. The photodiodes 15A and 15B are differentially connected to an operational (OP) amplifier 21-3 through high-input OP amplifiers 21-1 and 21-2, and a differential output is amplified by the OP amplifier 21-3. That is, an amplified output from the OP amplifier 21-3 is an output voltage V.sub.0, and it becomes a voltage V.sub.1 through a phase compensation circuit 22 inserted for improving response characteristics, and a servo amplifier 23. The voltage V.sub.1 is applied to the coils 14-1 and 14-2 of the torquer coil 17. Therefore, a current i is supplied to the coils 14-1 and 14-2, a force F.sub.2 for causing the displaced slit 12-1 to return to the zero point is generated by the torquer coil 17, and the force F.sub.2 balances with a force applied to the pendulum 12 by the acceleration. The current i at this time is detected by a current detection circuit 24 and appears as an output from an OP amplifier 24-1, and the input acceleration can be detected on the basis of the output from the OP amplifier 24-1.
According to the conventional servo accelerometer, as is obvious from equations (5) and (6), since accuracy is decided by a ratio of K to A.sub.0 A.sub.1 A.sub.2 A.sub.3, it is required to make the flexure member 11 thin so as to cause a spring constant K to be extremely small, and to have only one shaft having a degree of freedom to increase the accuracy. Therefore, the manufacture and assembly become difficult, and it is necessary to carefully handle the products.
In addition, in FIG. 7, a detection range used by the servo system is from Ls/2 to -Ls/2. When the pendulum 12 displaces beyond this range, positive feedback control is performed, and the pendulum 12 is kept swung to one side.
At this time (when the pendulum 12 is kept swung to one side), the pendulum 12 may be distorted beyond an elastic region of the flexure member 11. To prevent this distortion, stoppers (not shown) are arranged at both the sides of the pendulum 12. Since a commercially available two-split photodiode has an Ld dimension of 1 mm, both the stoppers must be adjusted to cause the pendulum 12 to swing within the range of 0.5 mm to -0.5 mm.
On the other hand, the pendulum 12 is designed so that the zero point of the displacement detection mechanism 18 is positioned in the vertical direction of a fulcrum of the pendulum 12 in an assembled state. When the spring constant K is not equal to zero (since the accuracy is set compromised with performance specifications of the products and the cost, K is not equal to 0), the pendulum 12 is not always positioned at the zero point of the displacement detection mechanism 18 because of an initial distortion or a distortion generated during assembly of the flexure member 11. That is:
(1) When the flexure member 11 is distorted, and the pendulum 12 is positioned within the detection range of the displacement detection mechanism 18, since the spring constant K is not equal to zero, an offset is included in the output from the accelerometer by the reaction force generated by the flexure member 11.
(2) When the flexure member 11 is distorted, and the pendulum 12 is positioned beyond the detection range of the displacement detection mechanism 18, as described above, the pendulum 12 is kept swung to one side.
Note that, when an assembly tolerance is large, a mechanism for finely adjusting a position of the two-split photodiode 15 is required. This adjustment can be performed either before a power source is turned on, or using an open feedback loop. In either case, the adjustment must be performed finely, so that it is inevitable to increase cost. In addition, since a position of the two-split photodiode 15 as an important functional part is adjusted, reliability of a servo accelerometer is degraded. Note that, to widen the detection range of the displacement detection mechanism 18, there is no technique except for making a chip size of the two-split photodiode 15 large, resulting in an increase in cost.
Moreover, since an output impedance of a photodiode is generally high, a circuit arrangement as shown in FIG. 8 is required. The high impedance amplifiers 21-1 and 21-2 such as Bi-MOS input OP amplifiers are used, and a signal line leakage must be prevented, resulting in an increase in cost.
Furthermore, although an LED is used as the light source 16 of the displacement detection mechanism 18, an intensity of an LED depends on temperature, and the intensity tends to be degraded. Since an incident light amount of the two-split photodiode 15 is changed with a change in intensity of an LED, a gain of the output voltage of a displacement detection circuit 21 varies, and an open gain of the servo accelerometer is varied. Therefore, a correction circuit for the change in intensity is required.